Prior art coating compositions that form surface protective coats on the surface of plastic substrates, typically organic resin substrates, for imparting a high hardness and mar resistance thereto include coating compositions obtained through (partial) hydrolysis of hydrolyzable organosilanes and coating compositions further comprising colloidal silica.
For example, JP-A 51-2736, JP-A 53-130732 and JP-A 63-168470 disclose coating compositions comprising an organoalkoxysilane, a hydrolyzate and/or partial condensate thereof, and colloidal silica, the alkoxy groups being converted in the presence of excess water into silanol. Coats resulting from these coating compositions are suitable for the protection of underlying substrates due to a high hardness and good weathering resistance, but lack toughness. Coats having a thickness of 10 μm or more are likely to crack during heat curing, upon removal from the heat curing furnace, during outdoor service, or upon abrupt temperature changes. Although buffered basic catalysts are selected as the curing catalyst in light of shelf stability, these coating compositions suffer from a problem which arises from the fact that the hydrolyzate and/or condensate of alkoxysilane is based on relatively low molecular weight compounds. In these relatively low molecular weight compounds, silanols having a very high reactivity are contained in large amounts. Such silanols undergo condensation reaction slowly even at normal temperature so that they convert to higher molecular weights with the lapse of time, resulting in coats having lower hardness. Further, some compositions have the problem of stability that they can gel and be no longer used for the coating purpose.
For overcoming these drawbacks, JP-A 9-71654 discloses a siloxane resin composition comprising a relatively large amount of silanol, which composition is shelf stable and forms a coat having a high hardness and flexibility. This siloxane resin is prepared by hydrolyzing an alkoxysilane with water, the amount of water for hydrolysis being an excess amount relative to the alkoxysilane (specifically 100 to 5,000 parts by weight of water relative to 100 parts by weight of alkoxysilane), and distilling off the alcohol resulting from hydrolytic condensation at a relatively low temperature of 80° C. or lower, for thereby controlling the proportion in RSiO3/2 units of those units having one silanol group represented by RSiO2/2(OH) wherein R is hydrogen or a substituted or unsubstituted, monovalent hydrocarbon group of 1 to 18 carbon atoms. Although the composition is improved in shelf stability, the use of excess water compromises the system efficiency and requires a complex removal step. The coat resulting from the composition is still insufficient in crack resistance and hardness. There is a need for further improvements in these factors.
With respect to the curing catalyst that finds a compromise between the high hardness of coats and the shelf stability of coating compositions, buffered basic compounds are often used, for example, alkali metal salts of carboxylic acids such as potassium formate, sodium acetate and potassium propionate; aminecarboxylates such as ammonium acetate, dimethylamine acetate, ethanolamine acetate, and dimethylaniline formate; and quaternary ammonium carboxylates, such as tetramethylammonium acetate, benzyltrimethylammonium acetate, and tetraethylammonium benzoate, as described in the above-cited patents. If non-buffered basic compounds including amines such as triethylamine, benzyltrimethylamine and pyridine, and alkali metal hydroxides and alkoxides such as sodium hydroxide and sodium methoxide are used, the resulting resins form coats which are susceptible to cracking despite a very high hardness, and are substantially low in shelf stability.